Internally treated plastic food containers

ABSTRACT

FOOD CONTAINERS OF NON-AROMATIC PLASTIC FILMS HAVING THEIR INTERNAL SURFACES SUITABLY SULFONATED AND NEUTRALIZED ARE DISCLOSED AS BEING USEFUL TO PACKAGE RAW OR PRECOOKED OLEAGINOUS FOODS SINCE THE FOODS CAN BE HEATED IN THE PACKAGE TO COOK IT OR WARM IT FOR SERVING WITHOUT RUPTURE OF THE PACKAGE.

3,705,041 INTERNALLY TREATED PLASTIC FOOD CONTAINERS Wilhelm E. Walles, Otis R. Mclntire, James K. Rieke, and Max V. Griggs, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Apr. 1, 1969, Ser. No. 812,299 I Int. Cl. B65b 25/22 US. Cl. 99-171 LP 9 Claims ABSTRACT OF THE DISCLOSURE Food containers of non-aromatic plastic films having their internal surfaces suitably sulfonated and neutralized are disclosed as being useful to package raw or precooked oleaginous foods since the foods can be heated in the package to cook it or warm it for serving without rupture of the package.

BACKGROUND OF INVENTION This invention relates to plastic food containers which have been internally treated to render the container resistant to fatty oils and greases.

It is well known that various foods are packaged in flexible polyolefin pouches, tubes, bags, and containers which are heat sealed or are reclosable by a closing device as illustrated by the Naito patent (3,198,228).

These food containers with their contents are normally frozen until it is desired to prepare them for the table. When polyolefin containers containing foods with fatty oils or greases, i.e., oleaginous foods, are defrosted or cooked in boiling water, the hot oil or molten grease dissolves in and causes the polyolefin film to collapse and disintegrate.

One way to overcome this problem has been to use cross-linked polyolefin resins to form the food bags or pouches. This solution to the problem requires the use of expensive irradiation equipment capable of emitting high energy electrons, X-ray, gamma rays, beta rays, etc. Also, one has to stay below about 10 million volts to avoid possible residual radioactivity in the treated film. Cross-linking by radiation does not work well for Saran and polyvinyl chloride where hydrogen chloride comes off and unzipping of the polymer occurs.

SUMMARY OF INVENTION It has now been found that non-aromatic polymer food containers can be given an internal sulfonation and neutralization treatment which is relatively cheap but effective to prevent the disintegration of the containers when they are heated during defrosting or cooking. It is sometimes desirable to sulfonate and neutralize both the inside and the outside of the food containers. If desirable, the package containing food can be heat-sterilized prior to freezing or to selling.

The present invention is thus useful in the food packaging art where oleaginous food can be cooked in the pouch or envelope in which it is packed and frozen. Examples of these frozen foods are fresh vegetables such as pears, string beans, etc. to which is added butter during the packing operation or just before cooking. The invention is also useful in the so called instant food field wherein precooked oleaginous foods such as fried potatoes, meats, seafood, or fowl are packaged and frozen to be later heated by the consumer in the package for quick meals.

The invention is further useful in the packaging of meats and cheeses in club packages, i.e., where the foodstuff is packed into a tube of plastic film which is subsequently heat sealed on both ends. These packages are sub- United States Patent r: CC

sequently sterilized or pasteurized before being stored or sold.

DETAILED DESCRIPTION The present invention relates to a food container made from a film envelope of a non-aromatic polymer or plastic in which the inner surface consists of a layer of the polymer containing a plurality of sulfonate salt groups which are suflicient in number to render the envelope resistant to fatty oils and which inherently also provides the envelope with reduced permeability to gases such as oxygen. The amount of these sulfonate salt groups ranges from about 0.001 to about 1.0 milligram sulfur trioxide equivalents per square centimeter of surface. The preferred range is from about 0.01 to about 0.1 milligram per square centimeter of surface.

This invention further relates to a method of packaging or sealing the aforementioned oleaginous foodstuffs in a package so that they can be subsequently heated in the same package to cook them or to Warm them for eating.

This invention is also characterized by an oleaginous foodstuff package comprising a sealed film envelope of a non-aromatic polymer wherein the inner surface comprises a layer of sulfonate salt groups sufficient to render the envelope resistant to fatty oils and an oleaginous foodstuff contained therein.

The terms food container comprising a film envelope and foodstuff package comprising a sealed film envelope" are intended to be generic to containers or packages for holding or retaining foodstuffs in which all or most of the outer surface comprises a film with only a small amount of the enevlope or package used for the closin device. The closing device per se forms no part of the present invention and can consist of any one of the well known devices in this field such as the aforementioned Naito closure, or molded plastic screw threads with a cap therefor joined to a tubular film envelope sealed on one end. If desired, the mechanical sealing device can be eliminated and the package or envelope can be heat sealed or adhesively sealed.

The non-aromatic polymers or plastics used in this invention are illustrated by homopolymers of olefins such as ethylene, propylene, isobutylene, butene-l; homopolymers of haloolefins such as vinyl chloride, vinylidene chloride; chlorinated polyolefins such as chlorinated polyethylene and chlorinated polypropylene; homopolymers of vinyl esters such as vinyl acetate, vinyl propionate and vinyl butyrate; homopolymers of acrylic acid esters and methacrylic acid esters such as ethyl acrylate, methyl acrylate, propyl acrylate, ethyl methacrylate, methyl methacrylate, propyl methacrylate; interpolymers of the monomers set forth above; and blends of the foregoing polymers.

The invention is also applicable to the above polymers which include a small amount of the usual additives such as pigments, antioxidants, plasticizers, antistatic agents, and the like.

The above plastic articles are sulfonated to the required degree by a vapor phase process using dilute gaseous sulfur trioxide, chlorosulfonic acid, or fluorosulfonic acid in an inert gas; by a liquid phase sulfonation process using dilute sulfur trioxide in an inert chlorinated solvent, concentrated sulfuric acid, or oleum. These techniques are Well known in the art as is shown by US. Pats. 2,945,842, 2,937,066 and 2,854,477. It is preferred to use sulfur trioxide (SO in the vapor phase since this method eliminates the problems of solvent recovery and water washing the treated substrates.

The plastic envelopes are contacted with the sulfonating agents for a period of time that varies inversely with the temperature and concentration in order to achieve the desired degree of sulfonation. For example, if one uses an 8 percent by volume concentration of S in an inert gas such as carbon dioxide at 25 C. to sulfonate the plastic envelopes to the extent of one milligram of S0 in the form of sulfonic acid groups per square centimeter of surface, one can use a contact time of about 30 seconds. This concentration can be reduced to 2 percent S0 if desired, with an increase of the contact time to about 2 minutes for the same degree of sulfonation. Other changes in the temperature, time, concentration, etc. can be made by those skilled in the art. It is preferred to use S0 in a concentration of 1-4 percent by volume in carbon dioxide gas for a period ranging from about ten seconds to about two minutes at 25 C.

Following sulfonation, the plastic envelopes containing sulfonic acid groups can be neutralized and bleached by contacting the sulfonated surfaces with an aqueous solution of an alkali metal hypochlorite to form a plurality of sulfonate salt groups. Other bleaching agents that can be used are alkali metal peroxides, perchlorate, chlorites, persulfates and perborates. Another method is to neutralize the sulfonated envelopes with gaseous or liquid ammonia or dilute solutions of non-bleaching inorganic and organic compounds. Examples of these are aqueous ammonia, aqueos amine solutions, aqueous solutions of alkali metal and alkaline earth hydroxide, carbonates, bicarbonates, acetates, etc. These neutralized envelopes are then bleached with the above bleaching agents or with gaseous chlorine dioxide, hydrogen peroxide solutions, and the like.

The above bleaching changes the brown color associated with the sulfonated polymer films to a light straw color which is more acceptable to the consumer. If desired, the bleaching step can be eliminated completely.

The foregoing sulfonation, neutralization and/ or bleaching steps can also be applied to the outside of the plastic envelopes, if desired, to prevent any deterioration of the envelope during heating due to accidental contact of the envelope with fats or oil beforehand.

The following examples are presented solely to illustrate this invention and are not to be construed as a limitation on the claims.

Examples 1-4 Four polyethylene bags seven inches square, heat sealed on three sides, with a molded integral closure device consisting of internal locking male and female ribs on the fourth side are sulfonated internally for the time and at the concentration of S0 in carbon dioxide specified in Table I. These bags are made from polyethylene film of 2 mils thickness. The bags are then immersed for one minute in an aqueous solution containing 2 percent by weight of ammonia and dried. The four treated bags and an untreated control are then opened and cubic centimeters of a liquid unsaturated vegetable oil (Crisco) along with 100 cubic centimeters of water are inserted to simulate the effect of an oleaginous food such as French fried potatoes. These bags are then squeezed to remove most of the air and sealed by means of the closure device. They are then placed in a beaker containing boiling water. The results are noted in Table I.

1 Time to the first indication oi leakage. 9 None at 60 minutes.

Example 5 A treated polyethylene bag of the type set forth in Example 3 is substantially filled with frozen peas, two tablespoons of butter is added, and the bag is sealed.

The bag containing the frozen peas with the butter is then immersed for seven minutes in boiling water. It is found that at the end of this time, the peas are completely cooked with the melted butter and no deterioration of the bag takes place.

Results similar to the foregoing are obtained with films made from the other non-aromatic polymers, ccpolymers, and blends thereof, listed above.

We claim:

1. A food container comprising a film envelope of a non-aromatic polymer, the inner surface thereof comprising a layer containing sulfonate salt groups sufficient to render the envelope resistant to fatty oils.

2. A food container as set forth in claim 1 in which the inner surface contains from about 0.001 to about 1.0 milligram sulfonate salt groups expressed as sulfur trioxide equivalents per square centimeter.

3. A food container as set forth in claim 2 in which the non-aromatic polymer is a polyolefin.

4. A method of packaging oleaginous foodstuffs in a package so that they can be subsequently heated in the same package which comprises sealing said foodstuffs in a film of a non-aromatic polymer, the inner surface of which comprises a layer containing sulfonate salt groups suificient to render the film resistant to fatty oils.

5. The method as set forth in claim 4 in which the inner surface contains from about 0.001 to about 1.0 milligram sulfonate salt groups expressed as sulfur trioxide equivalents per square centimeter.

6. The method as set forth in claim Sin which the nonaromatic polymer is a polyolefin.

7. An oleaginous foodstuff package comprising a sealed film envelope of a non-aromatic polymer, the inner surface thereof comprising a layer containing sulfonate salt groups sutficient to render the envelope resistant to fatty oils, and an oleaginous foodstuff contained therein.

8. The package as set forth in claim 7 in which the inner surface contains from about 0.001 to about 1.0 milligram sulfonate salt groups expressed as sulfur trioxide equivalents per square centimeter.

9. The package as set forth in claim 8 in which the non-aromatic polymer is a polyolefin.

References Cited UNITED STATES PATENTS 2,861,101 11/1958 Tousignant et al. 260-793 X 2,937,066 5/1960 Walles 260-949 C X 3,407,077 10/ 1968 Helin 99171 H 3,446,631 5/1969 Samuels 99-171 R 3,479,390 11/ 1969 Blatz et al 26079.3 UX 2,446,536 8/1948 Hardy 117-118 FRANK W. LUTTER, Primary Examiner S. L. WEINSTEIN, Assistant Examiner US. 01. X.R. 

